For substantially colorless high transmittance glass sheets, a high purity raw material is used so that an iron content is extremely low with respect to that of a conventional soda-lime glass sheet, thereby obtaining a light colored and high transmittance glass sheet.
For example, an edge coloration clear glass disclosed in JP 7(1995)-29810 B is a soda-lime glass that contains as a coloring agent, expressed in wt. %, less than 0.02% of total iron oxide in terms of Fe2O3, and has a ratio of ferrous oxide (FeO) to this total iron oxide of at least 0.4. This allows the glass to achieve low coloration and high transmittance. The glass has a luminous transmittance (illuminant C) of at least 87% on a 5.66 mm thickness basis.
In order to attain the above-mentioned properties, this glass has the following features. That is, in terms of a manufacturing method, the glass has a low SO3 content, and a melting operation includes a liquefaction stage and a refining stage as separate stages. Further, in terms of a material, a batch free from limestone and dolomite is used so that an iron content in the glass is lowered.
Furthermore, an edge coloration clear glass disclosed in JP 8(1996)-715 B is obtained by adding trace amounts of Se and CoO to a glass composition having an iron oxide content equivalent to that of the above-mentioned glass so that the glass exhibits a dominant wavelength of 570 to 590 nm that is compatible with wood tones.
In order to obtain a light color tone and high transmittance glass having an iron oxide content equivalent to that of a conventional glass, a method has been known in which an oxidizing agent such as cerium oxide is added so that the content of FeO is lowered, which causes coloration and a decrease in transmittance.
For example, JP 5(1993)-221683 A discloses a clear glass whose radiation light transmittance is regulated. The clear glass has a conventional clear soda-lime glass composition containing, expressed in wt. %, 0.06 to 0.12% of iron as an impurity in terms of Fe2O3. In the glass, 0.1 to 0.5% of CeO2 is contained as an oxidizing agent so that a ratio of Fe2+/Fe3+ in the glass is lowered from a ratio of about 38% in the conventional soda-lime glass sheet to 3 to 10%, thereby attaining high transmittance in a wavelength region in the vicinity of 600 nm or higher.
A method also has been proposed, in which a base composition of a soda-lime glass having a content of iron as an impurity equivalent to that of the conventional glass is changed so that lower coloration is attained.
For example, JP 8(1996)-40742 A discloses a clear glass composition for glass windows that is a soda-lime-silica glass composition containing, expressed in wt. %, a total amount of 0.02 to 0.2% of iron oxide in terms of ferric oxide and having a base composition that contains 69 to 75% of SiO2, 0 to 3% of Al2O3, 0 to 5% of B2O3, 2 to 10% of CaO, less than 2% of MgO, 9 to 17% of Na2O, 0 to 8% of K2O, and optionally, fluorine, zinc oxide, zirconium oxide, and less than 4% of barium oxide. In the glass composition, a total content of alkaline-earth metal oxides is not more than 10%. Thus, an absorption band of FeO is shifted to a longer wavelength side, or a slope of the absorption band by FeO is straightened at an infrared side end of the visible region. This allows window glasses to have lower coloration and exhibit more excellent infrared absorption than a soda-lime-silica glass having a conventional base composition.
The edge coloration clear glass disclosed in JP 7(1995)-29810 B is required to have a ratio of the ferrous oxide (FeO) to the total iron oxide of at least 0.4 so that a pure and bright azure color can be obtained as desired.
In order to attain this ratio, a particular manufacturing method in which the melting operation includes the liquefaction stage and the refining stage as separate stages is desirable, and the content of SO3 should be limited to a low level. This results in an excessive cost increase of the glass sheet thus obtained.
In the edge coloration clear glass disclosed in JP 8(1996)-715 B, Se and CoO are contained as coloring agents, thereby causing a decrease in transmittance. Accordingly, the glass is not suitable for applications requiring high transmittance.
In the clear glass disclosed in JP 5(1993)-221683 A, iron oxide contained in an amount equivalent to an iron oxide content of the conventional soda-lime glass sheet is oxidized by adding a required amount of an oxidizing agent such as cerium oxide, so that the ratio of Fe2+/Fe3+ of the contained iron oxide is made lower than that in the case of the conventional soda-lime glass sheet.
According to this method, by reducing the absorption of FeO, the absorption in a wavelength region having its peak at a wavelength in the vicinity of 1,000 nm can be reduced. However, the absorption is not reduced to a sufficient degree. Further, the absorption by Fe2O3 at a wavelength in the vicinity of 400 nm is increased, so that a color tone of the glass becomes yellowish. Thus, the glass is not preferred for use as a high transmittance glass sheet.
Furthermore, since the iron oxide is contained in an amount equivalent to the iron oxide content of the conventional soda-lime glass sheet, in order to lower the ratio of Fe2+/Fe3+, it is required that the oxidizing agent be used in a relatively large amount, thereby causing an increase in manufacturing cost of the glass sheet.
Moreover, since the absorption at a wavelength in the vicinity of 400 nm is increased as described above, when used as a substrate for a solar cell, which includes a photoelectric conversion layer of amorphous silicon having the highest sensitivity of energy conversion at a wavelength in the vicinity of 500 to 600 nm, the efficiency of the energy conversion is decreased.
In the glass composition disclosed in JP 8(1996)-40742 A, by changing the base composition of the glass, the soda-lime glass having an iron oxide content equivalent to that of the conventional glass is increased in transmittance.
However, the method disclosed in this publication merely can provide the effect of shifting the absorption of FeO to the longer wavelength side, which is insufficient for use for buildings in which no coloration is desired and applications requiring high transmittance.
Furthermore, in the composition disclosed in this publication, MgO and MgO+CaO are contained in insufficient amounts, respectively, and a resultant inconvenience related to melting is compensated by containing Na2O in a larger amount than in a conventional case. As a result, water resistance and weather resistance are deteriorated, and thus browning becomes more likely to be caused. Further, this composition is not suitable for mass production from the viewpoint of cost effectiveness.
Furthermore, although the effect disclosed in the publication can be enhanced by adding components such as F, BaO and the like, the addition of these components leads to a cost increase, shortening of a furnace life attributable to the volatilization of F, and release of harmful substances into the air.